The characteristics of biofuel are in many ways far superior to fossil fuels, generating much lower levels of pollutants (a 25/75% blend known as B-25 – reduces output of harmful emissions by almost 50% when compared to 100% kerosene) and having no “carbon footprint”, however the most significant operational difference between the two is the temperature at which they begin to solidify. Jet A is approved for use to -40 degrees centigrade (-40C), as the cloud point (the temperature at which the fuel begins to change from a liquid to a solid) falls below this temperature. Fuel temperatures experienced during the operation of jet aircraft typically range from well above freezing to around -25C, with colder temperatures being experienced after significant time at high cruise altitudes during the winter.
Depending upon the source material, biofuel cloud points can be as high as (0C), the operational effects are significant and must be considered when using a fuel of this type. Low percentage biofuel blends have a very similar cloud point to the “host” fuel and currently have significant practical applications, and are well proven in ground use.
The use of 100% biofuel is not currently a practical alternative to jet A, however given the current interest in the development of “alternative fuels” an excellent opportunity existed for the demonstration of its suitability for the use in jet aircraft once the temperature and viscosity limitations have been mitigated, either by further research and development or by blending with existing suitable fuel types.
The project was challenging to begin with, as no one at the time had performed adequate turbine engine performance tests with renewable fuel, much less demonstrated flight tests. The aircraft of choice was a modified 1968 Czechoslovakian L-29 Delfin, a.k.a. BioJet 1. The L-29, originally built as a military trainer, was the ideal aircraft for testing the use of biodiesel fuel in a jet aircraft because it features built-in fuel heaters, which alleviate the concern of fuel jelling. Additionally, the L-29 is rated to fly on a variety of fuels, including heating oil, which makes it an ideal platform for testing biodiesel in jet engines.
Flight Tests involved aircraft operations within the normal operational envelope, at speeds from Vso to 250KTS,and density altitudes from sea level to 18,000’. Engine operational performance on B100 fuel within this envelope was unknown, hazards associated with experimental fuel may be;
- Complete engine failure
- Insufficient thrust for level flight
- Reduced engine thrust, resulting in;
- increased takeoff distance.
- reduced climb performance.
- reduced altitude capability.
- Reduced cruise airspeed, range and endurance.
- Fuel starvation due to fuel line, filter or control unit blockage caused by fuel gelling or freezing, resulting in 1, 2, or 3 above.
- Loss of engine control or damage due to Fuel Control Unit incompatibility with biofuel.
- Engine damage due to increased Exhaust Gas Temperatures, loss of control or overspeed.
Flight Test hazards were mitigated by;
- Taking off from runways at least 25% longer than the minimum required by standard manufacturers airplane performance charts.
- Flying only within VISUAL meteorological conditions.
- Operating within safe gliding distance of the takeoff or other suitable airport.
- Conducting an engine run up/isolation valve test confirming isolation valve operation, and full power run up confirming 100% RPM operation and noting maximum EGT prior to each flight.
- Inspecting, cleaning and/or replacing the fuel filter at regular intervals.
- Observing maximum EGT limits and maximum climb and cruise RPM and EGT limits. Reducing start EGT limit by 10 degrees (690C), reducing maximum RPM by 1% during flight and close monitoring of engine performance indications.
Many prestigious research organizations have years of research and data establishing biofuel’s safe use in internal combustion engines. Green Flight International has demonstrated that it is also safe and practical for use in aircraft turbine engines. Throughout the program working with the fuel proved no different than working with Jet A given a few operational limitations.
The Green Flight international ground and flight test program proved successful after 18 months of R&D. The objective of establishing base line engine and airframe performance data using Jet A fuel and determining the impacts and limitations on flight operations of B25, B50 and B100 fuel led to F.A.A. approval for the historic 2,486 mile U.S. Transcontinental flight from Reno, Nevada to Leesburg, Florida on November 1, 2008.